For semiconductor lasers, there has been used the Fabry-Perot (FP) type, which constitutes an optical resonator sandwiched between mirrors formed on both end surfaces of an active layer. However, the FP type laser oscillates at a wavelength that satisfies the standing wave condition, and thereby tends to operate in a multi-longitudinal mode. Particularly, when current or temperature is changed, the oscillation wavelength varies so that the light intensity is changed.
For this reason, lasers are required to have higher wavelength stability and oscillate in a single mode in applications such as optical communication and gas sensing. To this end, a distributed feedback (DFB) laser and a distributed Bragg reflector (DBR) laser have been developed. These lasers are configured to include a diffraction grating in the semiconductor and to oscillate only at a specific wavelength by making use of the wavelength dependency thereof.
In order to realize a semiconductor laser having the wavelength stability, there may be exemplified a DBR laser and a DFB laser in which a grating is monolithically formed in a semiconductor laser, and an external cavity laser in which a fiber grating (FBG) is attached to the outside of the laser. These lasers are based on the principle that a portion of the laser light is returned to the laser by using a mirror having the wavelength selectivity utilizing the Bragg reflection to achieve a stable wavelength operation.
The DBR laser achieves a resonator by forming concave and convex portions at a waveguide surface on the extension of a waveguide of an active layer by the Bragg reflection. Since this laser is provided with diffraction gratings on the both ends of an optical waveguide layer, a light emitted from the active layer propagates through the optical waveguide layer, whereby a portion thereof is reflected by this diffraction grating and is then returned to a current injection part to be amplified. Since only a light having a specific wavelength is reflected in a predetermined direction from the diffraction grating, the wavelength of the laser light becomes constant.
Moreover, as this application, there have been developed an external cavity type semiconductor laser in which the diffraction grating is a component different from the semiconductor and a resonator is formed outside the semiconductor. The type of the laser results in a laser excellent in the wavelength stability, the temperature stability and the controllability. As the external resonator, there are a fiber Bragg grating (FBG) (non-Patent Document 1), and a volume hologram grating (VHG) (non-Patent Document 2). Since the diffraction grating is constituted as a member different from the semiconductor laser, there is such a feature that the reflectivity and the resonator length can be individually designed. Since it is not subjected to the influence of temperature elevation by heat generation due to current injection, the wavelength stability can be further improved. In addition, the diffraction grating may be designed in conjunction with the resonator length, because the change in the refractive index of the semiconductor depending on the temperature is different from that of the diffraction grating, thereby making it possible to enhance the temperature stability.
Patent Document 1 (Japanese Patent Publication No. 2002-134833A) discloses an external cavity laser utilizing grating formed at a quartz glass waveguide. This aims at providing a laser with the frequency stability, which can be used, without a temperature controller, under an environment where the room temperature greatly changes (e.g. 30° C. or higher). In addition, it describes that there is provided a temperature independent laser in which mode hopping is suppressed and there is no temperature dependency of the oscillation frequency.
As the optical device in which a light emitting device of such external cavity type is further coupled to an optical waveguide element or an optical fiber array, there are three methods described below.
In the method 1, a semiconductor laser incorporated grating element is optically and axially aligned with an optical waveguide element so that they are connected to each other (Patent Document 3).
In the method 2, a semiconductor laser light source and a grating-incorporated optical waveguide element or an optical fiber are coupled so that they are subjected to alignment between them (Patent Document 4, Patent Document 5).
In the method 3, three components of a semiconductor laser light source, a grating element and an optical waveguide element are optically coupled to each other (Patent Document 1).
However, in the method 3, alignment with the sub-micron accuracy is required, and it takes tact so that commercialization thereof is difficult. For this reason, there are proposed techniques to integrate the grating with the semiconductor laser, or the waveguide or the fiber as in the methods 1 and 2.